A comparative study will be made of cells in long-term tissue culture derived from normal persons and from persons with Bloom's syndrome, a rare autosomal recessive disorder of growth. Attention will be given to certain time-dependent abnormalities which appear in such cultures, including nuclear distortion and chromosome aberrations and the declining rate of proliferation. The rationale for such a comparative study is the following: Because many of the abnormalities which appear in normal cells after many generations of life in vitro appear earlier and are more accentuated in the genetically abnormal Bloom's syndrome cells, the significance of such abnormalities, particularly in respect to the concept of "cellular senescence", may become more apparent in the mutant than in normal cells.